1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using gettering technique. Particularly, the present invention relates to a method of manufacturing a semiconductor device using a crystalline semiconductor film fabricated through the addition of a metal element that catalyzes the crystallization of a semiconductor film.
In the present specification, the “semiconductor device” denotes devices in general that can function by utilizing semiconductor properties, and electro-optic devices, semiconductor circuits, and electronic equipment all are semiconductor devices.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as a “TFT”) has been known as a typical semiconductor device including a semiconductor film having a crystalline structure (hereinafter referred to as a “crystalline semiconductor film”). The TFT has been receiving attention as a technique of forming an integrated circuit on an insulating substrate such as glass, and for example, a driving circuit-integrated liquid crystal display device is proceeding toward practical utilization. In the conventional technique, an amorphous semiconductor film formed by a plasma CVD method or a low pressure CVD method is subjected to a heat treatment or a laser annealing process (a technique of crystallizing a semiconductor film by laser beam irradiation) and thereby a crystalline semiconductor film is fabricated.
The crystalline semiconductor film thus fabricated is an aggregation of many crystal grains crystal orientation of which is oriented in arbitral direction and the crystal orientation cannot be controlled, which constitutes a factor restricting the characteristics of the TFT. With respect to such a problem, Japanese Patent Application Laid-open No. Hei 7-183540 discloses a technique of fabricating a crystalline semiconductor film through the addition of a metal element such as nickel that catalyzes the crystallization of a semiconductor film. This technique not only provides an effect of lowering the heating temperature required for the crystallization but also can improve the orientation of crystal grains into a single direction. The formation of a TFT with such a crystalline semiconductor film allows not only electron field-effect mobility to be improved but also a subthreshold coefficient (S-value) to be lowered and thus makes it possible to improve electric characteristics by leaps and bounds.
However, since the metal element that exerts a catalytic action is added, there are problems in that the metal element remains in the crystalline semiconductor film or at its surface to cause variations in characteristics of the device obtained, and the like. One example of the problems is a problem such that an Off-state current increases in the TFT and varies among individual devices. In other words, once the metal element that catalyzes crystallization is used to form a crystalline semiconductor film, it becomes rather an unwanted existence.